ATP-binding properties of human Hsp90

Hsp90 is one of the most abundant proteins in the cytosol of eukaryotic cells. Under physiological conditions Hsp90 has been shown to play a major role in several specific signaling pathways, including maturation of various kinases and maintenance of steroid receptors in an activable state. It is well established that the level of Hsp90 increases severalfold under stress conditions, and it has been shown that the chaperone function of Hsp90 is ATP-independent. Although yeast Hsp90 does not bind ATP, as determined by a number of methods monitoring tight binding, ATP-dependent functions of Hsp90 in the presence of co-factors and elevated temperatures are still under discussion. Here, we have reinvestigated ATP-binding properties and ATPase activity of human Hsp90 under various conditions. We show that human Hsp90 does not bind ATP tightly and does not exhibit detectable ATPase activity. However, using electron spin resonance spectroscopy, weak binding of spin- labeled ATP analogues with half-maximal binding at 400 μM ATP was detected. The functional significance of this weak interaction remains enigmatic.